Broadband speeds in the UK are slowest during a 'rush hour' peak period between 7pm and 9pm. Photograph: Tim Graham/Getty Images

There is a virtual rush hour after the real one, and it slows down people who want to get things done there, too.

New research has found that broadband download speeds in the UK drop by an average of 35% during the evening as millions of technology users go online and find themselves competing for the same bandwidth on phone lines in their area.

The study by uSwitch.com analysed data from 2m broadband speed tests across Britain, and revealed a huge fluctuation in surfing speeds during peak and off-peak times.

Those who navigate the web during the morning hours of 2am and 3am get the quickest download times, with a fastest average download speed of 9.6Mbps, analysts found.

By contrast those who surf the web between 7pm and 9pm – the peak period when most people get home and go online – face the slowest times, with average speeds dropping by a third to 6.2Mbps.

The figures include averages for fibre-optic connections such as Virgin Media's cable systems and BT's Infinity connections, although neither is yet broadly used.

Most people use ADSL connections, which send and receive their signals over BT's copper wiring between their homes and the local exchange, and which have limited data capacity. At the exchange, the system generally links to high-speed connections into BT's "backbone" which has far higher speeds.

The slowdown in speed comes from "contention" between multiple households trying to get data over the same copper lines: as more people try to get online and send and receive data, the amount that can be sent to each individual connection hits a peak.

Most households are on connections with a "contention ratio" of 1:50, meaning that they may be sharing the connection to the exchange with up to 49 others.

During the day many households will be unoccupied, so those on the line will be able to use more of its capacity. But as more people come home, the available capacity falls.

The data, published by uSwitch, also uncovered huge regional variations in peak and off-peak speeds. Internet users in Evesham, Worcestershire, experienced a massive 69% drop, with an average morning download reading of 15.5Mbps, falling to 4.9Mbps in the evening.

Weston-Super-Mare in Somerset saw a 64% drop, with off-peak times plunging from 9.5Mbps to 3.4Mbps during peak hours.

In Wadebridge, Cornwall, web users were faced with relatively limp off peak speeds of 4.1Mbps, which halved to just 2.1Mbps in the evening.

The communications watchdog Ofcom said in March that internet users were still only getting half of the headline advertised speed, and in September said that the average speed was 6.2Mbps, rather than the claimed 13.8Mbps.

Ofcom recommended in September that ISPs should only advertise speeds that are in reach of at least 10% of customers – but now it may also have to specify a time of day for the testing.

Higher speeds are available with the replacement of copper lines with fibre-optic systems, which have far greater capacity and do not suffer from the same contention problem.

But BT has complained that it is too expensive to rewire the UK with fibre-optic and that it might not recoup the investment – which has led the government to examine other incentives, especially to bring fibre-optic connections to rural areas, which are seen as the most expensive to rewire, yet could benefit the most from higher speed connections by minimising the need for travel as fuel costs rise.

Ernest Doku of uSwitch said only a small number of internet users are consistently enjoying the maximum headline broadband speeds offered by providers.

"It really is surprising just how much broadband speeds fluctuate at different times of the day, with drop-offs of almost 70% in some areas of the UK," he said.

"This research may help to shed some light on why many bewildered consumers, who believe they've signed up to a certain broadband speed, never actually feel like their connection is fast enough.

"Although providers are working hard to upgrade the UK's broadband infrastructure, there is a long road ahead to ensure that everyone can enjoy a much more consistent service."

Mr Doku advised people to perform an online broadband speed test to ensure they are getting the best possible package for their area.